1. Field of the Invention
The invention pertains to the art of multiple compressor refrigeration systems of the type in which one of the compressors at least is subject to individual unloading.
2. Description of the Prior Art
While parallelly arranged compressors have been taught in the prior art as in U.S. Pat. Nos. 3,503,223, 3,386,262 and 2,253,623 for example, so far as we are aware they have not been used to any appreciable extent in the form of hermetic shell type compressors in which one of the two is subject to being partly unloaded so that the capacity of the system can be varied through four operating steps.
U.S. Pat. Nos. 3,785,169 and 3,775,995, assigned to the same assignee as this application, disclose multiple hermetic shell compressors, one of which is part unloading to obtain the four capacity steps, arranged in a way that all of the suction gas is returned first to the part unloading compressor and then passing part of the suction gas through the shell of the first compressor to the shell of the second, non-unloading compressor. This arrangement is successful in its commercial usage and is believed to be the only arrangement in which dual hermetic shell compressors, of which one is part unloading, satisfactorily provide the four operating capacities without unduly restricting the operating limits in terms of the saturated discharge temperatures.
The problem of the limited saturated discharge temperature at operations of less than full capacity stems from any reciprocating compressor which is unloaded tending to run hotter than in a fully loaded condition. This is because of the recirculation of the hot gas within the partly unloaded compressor. A two compressor arrangement in which one can be unloaded to half capacity while the other compressor runs at full load or not at all permits the four capacity steps of 100%, 75%, 50% and 25%. At the 75% level, the problem is probably at its worst in the sense that there is a reasonably high system load with high condenser temperatures and pressures. At the 25% system capacity level, even though the first or lead compressor is operating half unloaded, the lower system load and lower condenser temperatures and pressures tend to alleviate the problem. With the series or tandem arrangement of the last two noted patents, when the system is operating at the 75% capacity level the second or lag compressor is forced to receive its suction gas through the shell of the part unloaded first or lead compressor so that the heat generated by the bypassing of the gas in the lead compressor is dissipated to a degree by the flow of gas entering the fully loaded lag compressor. Also in this situation the lag compressor determines the saturated discharge condition limit for any given saturated suction condition, since its entering suction gas is receiving additional superheat from the unloaded compressor. From the foregoing it will be understood why the suction gas return arrangement of the last two noted patents results in the satisfactory operation in the various capacity steps.
While that arrangement is satisfactory from an operating standpoint, it is not wholly satisfactory with respect to requiring the use of two compressors which have different constructions, and in particular the use of a lead compressor which is not standard with respect to compressors which are to be used singly. Compressors of the type and size used in this invention are manufactured for use either singly, or as one of two compressors in a multiple system. Compressors of a given size will typically be built both in a non-unloading version, as well as an unloading version, since the customer of a single compressor of a given size may desire either one or the other, depending upon the system load characteristics. The unloading compressors are more expensive than the non-unloading compressor because of the additional mechanisms involved. Accordingly, with the arrangements of the last two noted patents of our assignee, the lead compressor is a compressor of a special version and is usable only as a lead compressor in the multiple compressor system of the patents. This is because it is built with the enlarged suction line entry port, and with the large suction gas exit port at the opposite side of the shell. Thus if a customer wishes to replace the lead compressor in that arrangement, he must have precisely that version of compressor and may not use a standard unloading type compressor of that particular size which is manufactured for use in a single compressor system. Or if a customer wishes to buy an unloading compressor of that particular size for use in a single compressor system, the compressor of that size with the enlarged suction gas inlet and the suction gas exit port is not considered as satisfactory.
Therefore, the problem with which this invention is concerned is to provide an arrangement in which the lead compressor for the multiple compressor system can be a standard unloading type compressor but in which the advantages of the tandem system of compressors is still basically available.